Basically I owned an s21 ultra for about a year however it got smashed about a week ago so I rang the insurance got it sorted and they sent me a restored one (not the same phone) I made sure to back everything up onto my Samsung cloud in order to restore it all back onto this device, however my gallery hasn't backed up, is there any way for me to retrieve that?
There is if you have already linked your one drive to your samsung account. A while ago now, samsung gallery stopped syncing with samsung cloud. Instead you would have been prompted to setup & sync with one drive.
First of all you should sign in to onedrive with the account that you used. If you haven't linked your samsung gallery to one drive as is usually prompted then you can do that later. Also check your Google photos app, that may also have backed up. Once you sign in to onedrive your photos/files should start to sync. You can choose to download them back on to your device if you want.